As the world population increases and large cities expand over crowded shores, offshore platforms have become an acceptable location for urban, strategic and commercial activities. In particular, offshore platforms can be used for a variety of applications, such as offshore islands that may support industrial buildings and dwelling houses, deep-water drilling equipment for scientific purposes, and oil and gas recovery installations. Offshore platforms can also be used as a location for offshore radar stations, airports, and other facilities for industrial activity and urban life.
In general, offshore platforms are divided into two groups, such as “fixed” platforms and “floating” platforms. Fixed platforms comprise an equipment deck, that is supported above the water by legs that extend down to and are seated on the sea floor. While relatively stable, such fixed platforms are typically limited to shallow waters, i.e., depths of about 150 meters or less. Floating platforms are typically employed in water depths of 150 meters and greater, and are held in position over the well site by mooring lines or chains anchored to the sea floor, or by motorized thrusters located on the sides of the platform, or by both. Although floating platforms are more complex to operate because of their greater movement in response to wind and wave conditions, they are capable of operating at substantially greater depths than fixed platforms. Floating platforms are also more mobile, and hence easier to move to other offshore well sites.
One of the known types of floating platforms is a platform of the floating barge type. Floating barges present a relatively large water plane area and are immersed slightly below the surface of the water, i.e. at a depth where wave action is most prevalent. The common problem with such barge platforms is that it is difficult to perform useful work from the decks thereof due to waves which wet the decks and which cause a rolling, pitching and yawing reaction of the vessel, rendering the barges unsatisfactory for most operations where stability of the platform is paramount.
Stability can be required, for example, when the floating platform is used as a location for offshore radar stations or when airplanes and helicopters land on the platform. To address such stability issues, so-called “semi-submersible” offshore platforms were designed that take into consideration that wind waves of ocean storms are relatively only surface disturbances of the ocean and do not create significant water movements at a depth of about 15 meters and greater. Conventional semi-submersible offshore platforms are used primarily in offshore locations where the water depth exceeds about 100 meters. This type of platform comprises a hull structure that has sufficient buoyancy to support the equipment deck above the surface of the water. The hull typically comprises one or more submersible “pontoons” that support a plurality of vertically upstanding struts or columns, which in turn support the deck above the surface of the water.
For example, U.S. Pat. No. 3,592,155 is directed to providing a flotation platform for diminishing its reaction to surface wave action and turbulence and is formed of a planar deck section supported by a plurality of buoyantly independent elongate wine bottle-shaped flotation modules cast of a homogeneous unitized material, such as concrete. Thus, the structure ensures a minimal adverse reaction to surface wave turbulence since the neck portion extending through the area of surface wave turbulence is of a small cross-sectional area.
Although such conventional offshore floating platforms can enhance stability of the platform impacted by relatively small waves with the height of less than about 5 meters, such structures cannot provide stable operation of the platforms, when they are affected by harsh weather conditions, such as oceanic storms and strong waves which are greater than 5 meters.